The invention relates to a transmission and/or reflection measuring method and apparatus, especially for the automatic final inspection or quality test of vacuum-deposited discs (plate glass) or similar substrates with respect to the uniformity of applied thin layers (coatings) and the maintenance of specified transmission and reflection tolerances for fixed wavelengths or different colors.
For obtaining specified optical properties, for instance, the refining of plate glass, suitable materials are applied on base bodies in thin films. The control of the transmission or reflection properties achieved is customarily carried out locally limited and off-line, usually by sampling. To this end, prepared samples or test specimens of the material to be characterized or tested are placed as a rule in the measuring volume of laboratory photometers. These photometers consist of optical arrangements with a light source which covers the required wavelength range; lenses for collimating and directing the measuring rays; a chopper, for instance, a chopper disc; optically a ray divider in the measuring ray; filters or optical grids for filtering out a predetermined measuring light wavelength behind the test specimen; detectors for generating a measuring signal proportional to the intensity of the measuring ray; and detector amplifiers for amplyifying the measuring signal of the detectors, and of a circuit for the subsequent analog signal evaluation. The use of such photometers for the on-line control of transmission or reflection on moving objects has not been possible to date.
Measurements at edge zones of the samples to be tested or test specimens are not possible. The local resolution in extensive measuring zones is only small. Because of the usually used a-c voltage amplifiers, the accuracy of the measurement is determined by the stability of the chopping frequency. This stability requirement can be met only with difficulty under industrial operating conditions.
Further limitations for conventional photometers in the production process are due to unavoidable soiling which affects the optical components, due to drift as a result of external influences such as temperature and variations, etc., as well as aging of lamps, detectors and other components.
It is an object of the invention to develop the photometric measurement of the transmission of or reflection at test specimens or measuring objects, especially coated glasses or substrates, in the range of detectable electromagnetic radiation, especially light, for determination or quality control of the samples to be tested, with respect to method and apparatus in such a manner that it becomes usable reliably over the long term under production conditions in on-line operation on moving objects.